The invention relates to a new type of raster scanning ion microscopes which provide both a determination of the lateral distribution of elements (isotopes) and compounds over large areas of the surface of specimens as well as a display of the surface topography.
In the invention the method of secondary ion mass spectrometry is used for analysis. This implies sputter erosion of the specimen by bombardment with primary ions at an energy of a few keV. Part of the atomic or molecular species emitted from the target is ionized (secondary ions) and thus can be analyzed quantitatively in a mass spectrometer. Presently used magnetic type secondary ion mass spectrometers of sophisticated design allow a determination of the lateral distribution of elements and compounds over small areas of the specimen surface, either by direct ion imaging as described by R. Castaing and G. Slodzian in Journal de Microscopie 1 (1962) 395 or by raster scanning ion imaging as reported by H. Liebl in the Journal of Applied Physics 38 (1967) 5277. The present state of the art has been reviewed by H. Liebl in the Journal of Physics E: Scientific Instruments 8 (--1975) 797.
In raster scanning ion microscopy the primary ion beam is focused to a microspot by use of a suitable lens and raster scanned over the specimen surface by means of two pairs of deflection plates. The secondary ions produced are analyzed by a mass spectrometer and recorded by a suitable detector system. The mass spectrometer is tuned to the desired mass number and the amplified output signal of the multiplier is used to modulate the beam intensity (z axis) in an oscilloscope whose x,y deflection is synchronized with the primary beam raster. The intensity distribution on the oscilloscope thus reflects the lateral concentration distribution of the element or compound considered. Moreover, continuous sputtering and repeated imaging provides a three-dimensional characterization of one or more elements in the sample. Instead of an oscilloscope other types of recorders such as multichannel analyzers may be used for recording.
Magnetic type secondary ion microscopes suffer from at least three drawbacks, (i) the surface area to be imaged or the field of view is restricted to a diameter or edge width of 0.4 mm at the most, (ii) the depth of focus is small, and (iii) the surface topography is not displayed. One of the reasons for these disadvantages is due to the fact that magnetic type mass spectrometers provide sufficient mass resolution only if the object and image size are small.